It is "highly possible" that a partial meltdown was occurring in one of the nuclear reactors damaged in Friday's powerful earthquake, a Japanese government spokesman said today, the most dire statement yet of the situation at the power plant.

Measures were taken at Unit 3 of the Fukushima Daiichi power plant today, including releasing radioactive air and injecting sea water to reduce pressure and cool the reactor down, to prevent a possible meltdown,
Chief Cabinet Secretary Yukio Edano said.

But he also said it is "highly possible" there has been a partial meltdown in the unit.

"Because it's inside the reactor, we cannot directly check it but we are taking measures on the assumption of the possible partial meltdown," he said, according to The Associated Press.

The government had earlier denied that there was any possible meltdown.

Today, according to the AP, Edano said the radiation around the reactor rose briefly above legal limits, but has since declined significantly, and he said the rods in the reactor were briefly exposed.

In all, cooling systems have failed at six of the reactors at two Fukushima nuclear plants, including the reactor that exploded, where observers and experts have also feared a meltdown could occur.

The extent of the damage a the reactors and the cause of the explosion, which led Japanese officials to order tens of thousands to evacuate the region around the plant, are not clear.

According to Japan's Nuclear and Industrial Safety Agency -- an independent body -- the only thing that could have caused the explosion was a meltdown of the reactor core.

Along with the uncertainty about the nuclear facilities, Japan continues to be shaken by aftershocks from the 8.9 magnitude earthquake that hit Friday.

The latest temblor, which had a magnitude of 6.2 according to the U.S. Geological Survey, was centered off the eastern coast, but closer to Tokyo than the quake that devastated parts of the country and triggered a tsunami that hit the west coast of North America.

The official death toll from Friday's earthquake and tsunami rose to 763, while local media reports put fatality totals closer to 1,300 people. With thousands unaccounted for in the hardest hit areas, that number is expected to rise.

Police said between 200 and 300 bodies were found along the coast in Sendai, the largest city in the area near the quake's epicenter.

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The explosion early Saturday ripped through one of the buildings on the Fukushima Daiichi plant, injuring four workers.

Radiation levels were high before the explosion -- at one point releasing as much radiation every hour as a person would normally absorb from the environment in a year -- but radiation outside the plant started decreasing after the blast and the pressure inside the reactor was also dropping, the spokesman said.

A government spokesman said the blast did not damage the nuclear reactor itself, which would cause radioactive material to leak out.

But Ryohei Shiomi with Japan's Nuclear Safety Commission said a meltdown was possible.

The evacuation radius has now been expanded to 20 kilometers (12 miles).

The majority of the 51,000 people living near the danger zone have already been evacuated, according to Shiomi.